Fourth Amendment: blood testing: driving while intoxicated (DWI): in drunk-driving investigations, the natural dissipation of alcohol in the bloodstream does not constitute an exigency in every case sufficient to justify conducting a blood test without a warrant; the principle that a warrantless search of the person is reasonable only if it falls within a recognized exception, see, e.g., United States v. Robinson, 414 U. S. 218, 224, applies here, where the search involved a compelled physical intrusion beneath McNeely’s skin and into his veins to obtain a blood sample to use as evidence in a criminal investigation; the State nonetheless seeks a per se rule, contending that exigent circumstances necessarily exist when an officer has probable cause to believe a person has been driving under the influence of alcohol because BAC evidence is inherently evanescent. Though a person's blood alcohol level declines until the alcohol is eliminated, it does not follow that the Court should depart from careful case-by-case assessment of exigency. When officers in drunk-driving investigations can reasonably obtain a warrant before having a blood sample drawn without significantly undermining the efficacy of the search, the Fourth Amendment mandates that they do so. See McDonald v. United States, 335 U. S. 451, 456. Circumstances may make obtaining a warrant impractical such that the alcohol's dissipation will support an exigency, but that is a reason to decide each case on its facts, as in Schmerber, not to accept the “considerable overgeneralization” that a per se rule would reflect, Richards v. Wisconsin, 520 U. S. 385, 393. Blood testing is different in critical respects from other destruction-of-evidence cases. Unlike a situation where, e.g., a suspect has control over easily disposable evidence, see Cupp v. Murphy, 412 U.S. 291, 296, BAC evidence naturally dissipates in a gradual and relatively predictable manner. Moreover, because an officer must typically take a DWI suspect to a medical facility and obtain a trained medical professional’s assistance before having a blood test conducted, some delay between the time of the arrest or accident and time of the test is inevitable regardless of whether a warrant is obtained; and though a blood test conducted in a medical setting by trained personnel is less intrusive than other bodily invasions, this Court has never retreated from its recognition that any compelled intrusion into the human body implicates significant, constitutionally protected privacy interests (U.S. S. Ct., 17.04.13, Missouri v. McNeely, J. Sotomayor).
Quatrième Amendement de la Constitution fédérale et analyse du sang dans le cadre de la conduite d'un véhicule automobile : le fait que le taux d'alcool dans le sang se dissipe au fil du temps ne constitue pas un motif permettant aux forces de police d'ordonner une prise de sang dans tous les cas sans warrant. La Cour n'entend pas ici se départir de sa jurisprudence selon laquelle c'est au cas par cas qu'il s'agit d'examiner si un warrant est nécessaire pour ordonner la prise de sang. Ainsi, dans les cas de conduite en état d'ébriété, si la police peut raisonnablement obtenir un warrant avant la prise de sang, cela sans porter préjudice à la preuve, le Quatrième Amendement oblige à demander ce warrant. Toute intrusion dans le corps humain met en jeu d'importants intérêts privés qui sont protégés par la Constitution fédérale.